Optical scanner and image forming apparatus including same

1. An optical scanner to scan a plurality of objects respectively with optical beams emitted from an optical beam emitter, the optical scanner comprising: a deflector assembly including at least one deflecting unit configured to deflect the optical beams in a main scanning direction; and a plurality of reflection optical systems each including, a plurality of reflection mirrors configured to reflect one of the optical beams, and a curvature adjustment unit including a holder structure configured to forcibly curve one of the plurality of reflection mirrors in a direction opposite the main scanning direction and a pressure unit configured to push the reflection mirror in a direction opposite the direction of the forcible curve, wherein the curvature adjustment unit adjusts a direction and an amount of curvature of a main scanning line on the object by adjusting an amount that the pressure unit deforms the reflection mirror, and a direction of a curve of the main scanning line on the object caused by the forcible curve of the reflection mirror formed by the holder structure is similar in the plurality of objects.

2. The optical scanner according to claim 1 , wherein at least one of the plurality of reflection optical systems is provided upstream of the deflector in a sub scanning direction, and remaining reflection optical systems are provided downstream of the deflector in the sub scanning direction.

3. The optical scanner according to claim 2 , wherein each of the plurality of reflection optical systems includes an identical number of reflection mirrors, and an alignment order of the reflection mirror to which the curvature adjustment unit is attached in a direction in which the optical beam travels is one of an odd number and an even number in the reflection optical systems provided upstream of the deflector in the sub scanning direction and the other of the odd number and the even number in the reflection optical systems provided downstream of the deflector in the sub scanning direction.

4. The optical scanner according to claim 1 , wherein the deflector assembly comprises a plurality of the deflecting units, the plurality of reflection optical systems are divided into a plurality of reflection-deflection groups each provided with one of the deflecting units, and relative position of the deflector and each of the reflection optical systems in a sub scanning direction is similar in all of the reflection-deflection groups.

5. The optical scanner according to claim 1 , wherein the pressure unit deforms the reflection mirror in each of the plurality of reflection optical systems to cause the main scanning lines on the objects to curve in a similar direction.

6. The optical scanner according to claim 1 , further comprising an inclination adjuster attached to one of the plurality of reflection mirrors, configured to adjust an inclination of the reflection mirror.

7. An optical scanner to scan a plurality of objects respectively with optical beams emitted from an optical beam emitter, the optical scanner comprising: a deflector assembly including at least one deflecting unit configured to respectively deflect the optical beams in a main scanning direction; and a plurality of reflection optical systems each including, a plurality of reflection mirrors to reflect one of the optical beams, and an even number of curvature adjustment units each including a holder structure configured to forcibly curve one of the plurality of reflection mirrors in a direction opposite the main scanning direction and a pressure unit configured to push the reflection mirror in a direction opposite the direction of the forcible curve, wherein the curvature adjustment unit adjusts a direction and an amount of curvature of a main scanning line on the object by adjusting an amount that the pressure unit deforms the reflection mirror, and in each of the reflection optical systems, a first half of the curvature adjustment units and a second half thereof curve the main scanning line in directions opposite to each other by respectively curving the reflection mirrors forcibly with the holder structures.

8. The optical scanner according to claim 7 , wherein, in each of the reflection optical systems, the first half of the curvature adjustment units are respectively provided on the reflection mirrors whose alignment order in a direction in which the optical beam travels are odd, and the second half of the curvature adjustment units are respectively provided on the reflection mirrors whose alignment order in the direction in which the optical beam travels is even.

9. The optical scanner according to claim 7 , wherein each of the pressure unit deforms the reflection mirror in each of the plurality of reflection optical systems to cause the main scanning lines on the objects to curve in a similar direction.

10. The optical scanner according to claim 7 , further comprising an inclination adjuster attached to one of the reflection mirrors, configured to adjust an inclination of the reflection mirror.

11. An image forming apparatus comprising: a plurality of image carriers; an optical scanner configured to form electrostatic latent images on the plurality of image carriers through optical scanning, respectively, comprising a deflector assembly including at least one deflecting unit configured to respectively deflect the optical beams in a main scanning direction and a plurality of reflection optical systems; a plurality of developing units each configured to develop the electrostatic latent image formed on one of the image carriers; and a plurality of transferers each configured to transfer the developed image on one of the image carriers to a transfer member, wherein each of the reflection optical systems includes a plurality of reflection mirrors to reflect one of the optical beams and an even number of curvature adjustment units each including a holder structure configured to forcibly curve one of the plurality of reflection mirrors in a direction opposite the main scanning direction and a pressure unit configured to push the reflection mirror in a direction opposite the direction of the forcible curve, each of the curvature adjustment units adjusts a direction and an amount of curvature of a main scanning line on the image carrier by adjusting an amount that the pressure unit deforms the reflection mirror, and in each of the reflection optical systems, a first half of the even number of curvature adjustment units and a second half of the curvature adjustment units curve the main scanning line in directions opposite to each other by respectively curving the reflection mirrors forcibly with the holder structures.